1. Field of the Invention
This invention relates to a stimulable phosphor sheet. This invention particularly relates to a stimulable phosphor sheet for use in an energy subtraction processing technique for radiation images, wherein an image signal is obtained which represents an image of a specific structure or part of an object represented by the radiation images.
2. Description of the Related Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is firstly exposed to radiation, which carries image information of an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed, and the processed image signal is utilized for reproducing a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced in the form of a hard copy or may be displayed on a display device, such as a cathode ray tube (CRT) display device. In the radiation image recording and reproducing systems, the stimulable phosphor sheet temporarily stores the radiation image, such that a final visible image can be reproduced on a final recording medium. For the sake of economy, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
In order that the stimulable phosphor sheet may be reused in the manner described above, any energy remaining on the stimulable phosphor sheet after the image signal has been detected from the stimulable phosphor sheet may be erased. For such purposes, as disclosed in, for example, U.S. Pat. No. 4,400,619 or Japanese Unexamined Patent Publication No. 56(1981)-12599, the stimulable phosphor sheet may be exposed to erasing light or heat, and energy remaining on the stimulable phosphor sheet may thereby be released. The erased stimulable phosphor sheet can then be reused to record a next radiation image.
Also, subtraction processing techniques for radiation images have heretofore been known. With the subtraction processing techniques for radiation images, an image corresponding to a difference between a plurality of radiation images of an object, which have been recorded under different conditions, is obtained. Specifically, a plurality of the radiation images, which have been recorded under different conditions, are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals, which image signal components represent the image information recorded at corresponding sampling points in the radiation images, are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing technique or the so-called energy subtraction processing technique. In the temporal subtraction processing technique, in order for the image of a specific structure of an object (for example, in cases where the object is a human body, the image of a blood vessel) to be extracted from the image of the whole object, the image signal representing a radiation image, which has been obtained without injection of contrast media, is subtracted from the image signal representing a radiation image, in which the image of the specific structure (for example, the blood vessel) of the object has been enhanced by the injection of contrast media. In the energy subtraction processing technique, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation having different energy distributions. Specifically, an object is exposed to several kinds of radiation with different energy distributions. Alternatively, the energy distribution of the radiation, which carries image information of an object, is changed after the radiation has been irradiated onto one of a plurality of stimulable phosphor sheets, after which the radiation impinges upon the second stimulable phosphor sheet. In this manner, a plurality of radiation images, in which different images of a specific structure are embedded, are formed with the radiation having different energy distributions. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure.
The subtraction processing is extremely effective, particularly for medical diagnosis, and electronics research has continued to develop improved subtraction processing techniques.
In the aforesaid radiation image recording and reproducing systems utilizing a stimulable phosphor sheet, the radiation image stored on the stimulable phosphor sheet is read out directly as an electric image signal. Therefore, with the radiation image recording and reproducing systems, the subtraction processing described above can readily be carried out. In cases where the energy subtraction processing is to be carried out, radiation images may be stored on two stimulable phosphor sheets such that the parts of the radiation images, which parts correspond to a specific structure, are different in the two radiation images. For such purposes, a technique may be employed, wherein the operation for recording a radiation image is performed two times with two kinds of radiation having different energy distributions. Alternatively, a technique may be employed, wherein a filter, which is constituted of a metal, or the like, and which absorbs low energy components of radiation, is located between two stimulable phosphor sheets, and the two stimulable phosphor sheets are simultaneously exposed to radiation, which carries the image information of the object. However, with the former technique, the problems occur in that the operation for recording the radiation image must be performed two times, and an artifact is apt to occur due to movement of the object between the two radiation image recording operations. Also, with the former technique, radiation irradiating means must be adjusted for each of the radiation image recording operations, and therefore the operation characteristics are bad. With the latter technique, the problems occur in that the filter must be located between the stimulable phosphor sheets, and therefore the processing of the stimulable phosphor sheets at the time of the image recording and the image readout is not easy to perform.
In order for the aforesaid problems to be eliminated, there has been proposed a stimulable phosphor sheet comprising a substrate constituted of a substance, which absorbs low energy components of radiation, and stimulable phosphor layers, which are overlaid respectively on the two surfaces of the substrate. Such a stimulable phosphor sheet is disclosed in, for example, U.S. Pat. No. 4,855,598. In cases where the proposed stimulable phosphor sheet is exposed to radiation carrying image information of an object, radiation images of the object are stored respectively on the two stimulable phosphor layers of the stimulable phosphor sheet. Scanning with the stimulating rays is performed successively on each of the two surfaces of the stimulable phosphor sheet, and light emitted by one of the two surfaces of the stimulable phosphor sheet and light emitted by the other surface of the stimulable phosphor sheet are successively detected photoelectrically. Two image signals are thereby obtained successively. A subtraction process is then performed on the two image signals. In this manner, a subtraction image can be efficiently obtained with a single image recording operation.
With the stimulable phosphor sheet proposed in U.S. Pat. No. 4,855,598, the stimulable phosphor layers are overlaid respectively on the two surfaces of the substrate. However, in cases where the image signals are to be detected from the proposed stimulable phosphor sheet, the two surfaces of the stimulable phosphor sheet must be scanned successively with two independent scanning operations, and the light emitted by one of the two surfaces of the stimulable phosphor sheet and the light emitted by the other surface of the stimulable phosphor sheet must be detected successively. Therefore, the operation characteristics of the image read-out operations are bad. Also, it is necessary for position matching between the thus obtained two image signals to be performed. The position matching between the two image signals is not easy to perform. Accordingly, it may be considered to locate two independent stimulating ray sources respectively on the sides of the two surfaces of the stimulable phosphor sheet and to simultaneously perform the image readout from the two surfaces of the stimulable phosphor sheet. However, in such cases, the size of the radiation image read-out apparatus for performing the image read-out operation cannot be kept small, and the cost of the radiation image read-out apparatus cannot be kept low.
In order for the problems described above to be solved, an energy subtraction processing method for radiation images, wherein radiation images are recorded with a single image recording operation and a subtraction image is capable of being obtained with a single image read-out operation utilizing a single stimulating ray source, and a stimulable phosphor sheet for use in the energy subtraction processing method have been proposed in, for example, Japanese Unexamined Patent Publication No. 6(1994)-130197. With the proposed energy subtraction processing method, a stimulable phosphor sheet comprising a substrate (an energy distribution separating filter layer) constituted of a substance, which absorbs low energy components of radiation and transmits at least stimulating rays having predetermined wavelengths, and stimulable phosphor layers, which are overlaid respectively on two surfaces of the energy distribution separating filter layer, is utilized. Also, with a single image recording operation, radiation images are stored on the stimulable phosphor layers and with radiation having different energy distributions. Thereafter, two image signals are obtained with a technique for detecting light emitted from two surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the two surfaces of the stimulable phosphor sheet, which technique is disclosed in, for example, U.S. Pat. No. 4,346,295. Specifically, in accordance with the disclosed technique for detecting light emitted from two surfaces of a stimulable phosphor sheet and thereby detecting two image signals from the two surfaces of the stimulable phosphor sheet, stimulating rays are produced by a single stimulating ray source and caused to scan one surface of the stimulable phosphor sheet, and light emitted from one of the stimulable phosphor layers of the stimulable phosphor sheet and light emitted from the other stimulable phosphor layer are simultaneously detected. A subtraction process is then performed on the thus obtained two image signals. With the energy subtraction processing method proposed in Japanese Unexamined Patent Publication No. 6(1994)-130197, the time required to read out the radiation images having been stored on the stimulable phosphor layers can be kept short. Also, a subtraction image having a high resolution can be obtained efficiently from the two image signals, which represent the radiation images such that no deviation in position occurs between the radiation images.
In Japanese Unexamined Patent Publication No. 6(1994)-130197, as the substance employed in the energy distribution separating filter layer of the stimulable phosphor sheet, which substance absorbs low energy components of radiation and transmits light having predetermined wavelengths, a substance comprising a polymeric binder and fine particles of lead glass, PbO2, or TiO2 dispersed in the polymeric binder is exemplified. The energy distribution separating filter layer absorbs the low energy components of radiation and transmits the light having wavelengths falling within the wavelength range of the stimulating rays. Therefore, it is firstly necessary for the energy distribution separating filter layer to have a high capability of separating energy distributions. Also, as described above, the stimulable phosphor sheet is used repeatedly to record a radiation image. Therefore, it is desired that the substance, which absorbs low energy components of radiation and transmits light having predetermined wavelengths, undergoes no change in characteristics with respect to the stimulating rays and the light, which is emitted by the stimulable phosphor sheet, when the stimulable phosphor sheet is used repeatedly, i.e. that the substance is chemically stable.
However, in cases where PbO2 is employed in the energy distribution separating filter layer, PbO2 decomposes little by little and becomes colored. Therefore, PbO2 is not suitable for long-term use. Also, it is not easy to ensure long-term chemical stability of PbO2, and PbO2 deteriorates little by little with the passage of time. As a result, PbO2 absorbs a laser beam acting as the stimulating rays or the light emitted by the stimulable phosphor sheet. In such cases, the problems occur in that a stimulable phosphor layer of the stimulable phosphor sheet (in particular, the stimulable phosphor layer formed on the surface of the energy distribution separating filter layer, which surface is opposite to the stimulating ray irradiation side surface of the energy distribution separating filter layer) cannot be stimulated sufficiently, or that the light emitted by the stimulable phosphor sheet does not emanate sufficiently from the surface of the stimulable phosphor sheet. Accordingly, in cases where an energy subtraction image is formed from the image signals having been detected from the two surfaces of the stimulable phosphor sheet, it often occurs that an energy subtraction image having good image quality cannot be obtained and a medical diagnosis cannot be made accurately.
In cases where TiO2 is employed in the energy distribution separating filter layer, if the thickness of the energy distribution separating filter layer is small, a high capability of separating energy distributions cannot be obtained. Also, if the thickness of the energy distribution separating filter layer is large, a high capability of separating energy distributions can be obtained. However, in such cases, due to the large thickness of the energy distribution separating filter layer, the intensity of the laser beam reduces markedly when the laser beam passes through the energy distribution separating filter layer. Therefore, the laser beam cannot readily reach the stimulable phosphor layer formed on the surface of the energy distribution separating filter layer, which surface is opposite to the stimulating ray irradiation side surface of the energy distribution separating filter layer. As a result, light having a high intensity cannot be emitted from the stimulable phosphor layer formed on the surface of the energy distribution separating filter layer, which surface is opposite to the stimulating ray irradiation side surface of the energy distribution separating filter layer. In addition, since the thickness of the energy distribution separating filter layer is large, the laser beam diffuses markedly when the laser beam passes through the energy distribution separating filter layer. Accordingly, an image having a high sharpness cannot be obtained from the image signal detected from the stimulable phosphor layer formed on the surface of the energy distribution separating filter layer, which surface is opposite to the stimulating ray irradiation side surface of the energy distribution separating filter layer. Thus as in the cases of PbO2, in cases where an energy subtraction image is formed from the image signals having been detected from the two surfaces of the stimulable phosphor sheet, it often occurs that an energy subtraction image having good image quality cannot be obtained and a medical diagnosis cannot be made accurately.
The primary object of the present invention is to provide a stimulable phosphor sheet, wherein an energy distribution separating filter layer, which is chemically stable, which has a high capability of separating energy distributions, and which acts such that an energy subtraction image having good image quality is capable of being obtained from energy subtraction processing, is employed as an intermediate layer between stimulable phosphor layers.
The present invention provides a first stimulable phosphor sheet, comprising:
i) an intermediate layer constituted of a substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, and
ii) stimulable phosphor layers, which are overlaid respectively on two surfaces of the intermediate layer,
wherein the substance contains a bismuth compound.
In the first stimulable phosphor sheet in accordance with the present invention, the bismuth compound should preferably contain at least one compound selected from the group consisting of halides of bismuth, oxides of bismuth, and the like. For example, the bismuth compound should preferably contain at least one compound selected from the group consisting of BiF3 (bismuth fluoride), BiOCl (bismuth oxychloride), Bi2(WO4)3 (bismuth tungstate), Bi12GeO20 (BGO), and Bi12SiO20 (BSO). As the bismuth compound, one of the above-enumerated bismuth compounds may be used alone. Alternatively, two or more of the above-enumerated bismuth compounds may be used in combination. Also, the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, may contain a substance other than the bismuth compound.
The present invention also provides a second stimulable phosphor sheet, comprising:
i) an intermediate layer constituted of a substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, and
ii) stimulable phosphor layers, which are overlaid respectively on two surfaces of the intermediate layer,
wherein the substance contains at least one lead compound selected from the group consisting of PbF2 (lead fluoride), 2PbCO3.Pb(OH)2 (basic lead carbonate), PbTe (lead telluride), and PbWO4 (lead tungstate).
In the second stimulable phosphor sheet in accordance with the present invention, the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, may contain one of the above-enumerated lead compounds alone. Alternatively, the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, may contain two or more of the above-enumerated lead compounds in combination. Also, the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, may contain a substance other than the lead compound.
As described above, each of the first and second stimulable phosphor sheets in accordance with the present invention comprises the intermediate layer and the stimulable phosphor layers respectively overlaid on the two surfaces of the intermediate layer. The first and second stimulable phosphor sheets in accordance with the present invention may also be provided with other layers having functions for a protective layer, an adhesion layer, a light reflecting layer, and the like.
Also, in the first and second stimulable phosphor sheets in accordance with the present invention, besides the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, the intermediate layer may contain other substances having other functions, such that the functions of the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, are not affected adversely. For example, the intermediate layer may also act as a substrate for the stimulable phosphor sheet.
The term xe2x80x9csubstance absorbing low energy components of radiationxe2x80x9d as used herein means the substance, which absorbs more of the low energy components of radiation than high energy components of the radiation. The definition of which energy components of radiation fall within the range of the low energy components of the radiation and which energy components of the radiation fall within the range of the high energy components of the radiation depends upon whether the radiation is utilized in a medical field or other fields. Also, in the medical field, the aforesaid definition depends upon image recording conditions, objects, radiation sources, and the like. Energy components of certain levels are defined as the low energy components in certain cases and are defined as the high energy components in the other cases. For example, as for the recording of ordinary chest images utilized in the medical field, the definition is roughly made such that the energy components of levels falling within the range of 10 keV to 50 keV are the low energy components and the energy components of levels falling within the range of 40 keV to 120 keV or higher are the high energy components.
The term xe2x80x9clight having predetermined wavelengthsxe2x80x9d as used herein means the light having wavelengths falling within the wavelength range of the stimulating rays, with which the stimulable phosphor sheet is scanned ordinarily, the wavelength range of the stimulating rays is from 500 nm to 800 nm. Such that the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, may not absorb the light emitted by the stimulable phosphor sheet, the substance should preferably be capable of transmitting light having wavelengths falling within the range of 300 nm to 800 nm.
The first and second stimulable phosphor sheets in accordance with the present invention should preferably be utilized for an energy subtraction processing technique.
The energy subtraction processing technique is a technique for processing radiation images, wherein a plurality of radiation images of a single object are formed with radiation having different energy distributions by utilizing the characteristics such that a specific structure of the object has different levels of radiation absorptivity with respect to the radiation having different energy distributions. Thereafter, a plurality of radiation image signals, which represent the radiation images of the object, are weighted, and the weighted image signals are subtracted from each other. A subtraction image signal is thus obtained, and an image, in which only the pattern of the specific structure of the object is illustrated or enhanced, is obtained from the subtraction image signal. (The energy subtraction processing technique is described in, for example, Japanese Unexamined Patent Publication Nos. 57(1982)-045475 and 59(1984)-083486.)
By way of example, the energy subtraction processing technique may be performed in the manner described below. Specifically, radiation is irradiated to an object, and radiation images of the object are stored on stimulable phosphor layers of a stimulable phosphor sheet, which comprises a substrate and the stimulable phosphor layers overlaid respectively on two surfaces of the substrate, such that image information of the object is formed on the stimulable phosphor layer, which is formed on the substrate surface opposite to the object, with radiation, in which low energy components of the radiation have been reduced at a region corresponding to a specific structure of the object to a level lower than in the radiation impinging upon the stimulable phosphor layer formed on the substrate surface on the side of the object. Thereafter, stimulating rays having predetermined wavelengths are caused to scan one surface of the stimulable phosphor sheet, and light emitted from one of the stimulable phosphor layers of the stimulable phosphor sheet and light emitted from the other stimulable phosphor layer are photoelectrically detected from the two surfaces of the stimulable phosphor sheet. In this manner, two digital image signals, which respectively represent the radiation images having been stored on the stimulable phosphor layers of the stimulable phosphor sheet, are obtained. The image signal components of the two digital image signals, which image signal components represent corresponding pixels in the two radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of the specific structure of the object represented by the radiation images. The energy subtraction processing technique performed in the manner described above is disclosed in, for example, Japanese Unexamined Patent Publication No. 6(1994)-130197.
In the subtraction process performed on the digital image signals, the image signal components of the digital image signals, which image signal components represent corresponding pixels in the radiation images, are weighted by being multiplied by weight factors, and the thus weighted image signals components are subtracted from each other to obtain the difference signal.
Each of the first and second stimulable phosphor sheets in accordance with the present invention comprises the intermediate layer constituted of the substance, which absorbs low energy components of radiation and transmits at least light having predetermined wavelengths, and the stimulable phosphor layers, which are overlaid respectively on the two surfaces of the intermediate layer. The substance contains the bismuth compound, which is chemically stable, or at least one lead compound, which is chemically stable and is selected from the group consisting of PbF2, 2PbCO3.Pb(OH)2, PbTe, and PbWO4. Therefore, with the first and second stimulable phosphor sheets in accordance with the present invention, the problems do not occur in that the substance decomposes little by little and the intermediate layer becomes colored. The substance undergoes little chemical change with the passage of time. Accordingly, the problems do not occur in that the substance absorbs a laser beam acting as the stimulating rays or the light emitted by the stimulable phosphor sheet. As a result, the problems do not occur in that a stimulable phosphor layer of the stimulable phosphor sheet (in particular, the stimulable phosphor layer formed on the surface of the intermediate layer, which surface is opposite to the stimulating ray irradiation side surface of the intermediate layer) cannot be stimulated sufficiently, or that the light emitted by the stimulable phosphor sheet does not emanate sufficiently from the surface of the stimulable phosphor sheet. Thus the first and second stimulable phosphor sheets in accordance with the present invention undergo little change in characteristics with respect to the stimulating rays and the light, which is emitted by the stimulable phosphor sheet.
Also, in cases where TiO2 is employed in the intermediate layer, the problems occur in that, if the thickness of the intermediate layer is small, a high capability of separating energy distributions cannot be obtained. However, the bismuth compound or the lead compound, which is selected from the group consisting of PbF2, 2PbCO3.Pb(OH)2, PbTe, and PbWO4, has a high capability of separating energy distributions. Therefore, with the first and second stimulable phosphor sheets in accordance with the present invention, even if the thickness of the intermediate layer is small, a high capability of separating energy distributions can be obtained. Accordingly, the thickness of the intermediate layer need not be set at a large value in order to obtain a high capability of separating energy distributions. As a result, the problems can be prevented from occurring in that the laser beam cannot readily reach the stimulable phosphor layer formed on the surface of the intermediate layer, which surface is opposite to the stimulating ray irradiation side surface of the intermediate layer, and light having a high intensity cannot be emitted from the stimulable phosphor layer formed on the surface of the intermediate layer, which surface is opposite to the stimulating ray irradiation side surface of the intermediate layer. Further, the problems can be prevented from occurring in that the laser beam diffuses markedly when the laser beam passes through the intermediate layer having a large thickness.
Accordingly, with the first and second stimulable phosphor sheets in accordance with the present invention, in cases where an energy subtraction image is formed from the image signals having been detected from the two surfaces of the stimulable phosphor sheet, an energy subtraction image having good image quality with a high sharpness can be obtained.